Our recent work has provided a serological definition of antigens specified by embryonic lethal genes at the T-locus in the mouse. The T-locus region has long been known to contain an important center for controlling both the organization and viability of the early embryo and the formation and function of spermatozoa. The fact that we can now detect on spermatozoa and on embryos the products of alleles in this region gives us a powerful new tool which may be crucial in reaching a new level of understanding of the mechanisms of embryonic morphogenesis and cytodifferentiation. We will characterize developing embryos by their representation of the genetically determined cell surface antigens specified by the morphogenetically important alleles at the T-locus, and also study them with respect to the appearance of H-Y and H-2 antigens. The functional role of these antigens during early development will be assayed. The ontogeny of the same antigens will be studied on spermatogenic cells as they mature into spermatozoa, and their topography mapped in these polarized cells. We have already demonstrated that spermatozoa containing the Y-chromosome can be selectively eliminated from the total population of spermatozoa by the use of anti- Y antisera; we will use similar but improved methods to alter the fertilizing capacity of specific genetic classes of sperm from males heterozygous for T-locus alleles.